You should use the Edge ID rather than the Arcane Background ID, so edgArcMir. Also I'm not sure if spaces work in Sources, I would have used Space1889 or just 1889 for a source, but as long as you're consistent it shouldn't really matter.
For handling cash modifications... usually if you use a modifier things should work fine. Take a look at the DLNoir.user file for some examples. Essentially if you have one hindrance do the 50% thing and removing a point, the other hindrance can do just the 50% again part without doing anything about points (just don't include that code), but I'm not sure how those hindrances work until I can take a look at them later to see exactly what you mean.
EDIT: I should probably add that you don't need to Preclude the Arcane Background, just the Edge, because the Background is never really seen anyplace other than through the Edge. For the moment you also wouldn't need to Preclude the Spellcasting skill, either, since you never see that unless the Background is taken, but some recent rules clarifications have shown that the skills should be be hidden for those because it is possible to have the skill without having the associated AB (so as to be able to use Weird Science gadgets, for example, or to activate a magic item that might require Spellcasting to know how to use or even to add your skill as a Cooperative Roll to someone else's skill.)